The influence of shape of suction nozzle on the measured particle concentration due to the anisokinetic sampling in duct is numerically analyzed by taking account of the viscous effect of a fluid. As a result, predicted results agree well for Psi > 1 with those obtained from potential theory. But instability of the measured concentration is observed when the inertia parameter is smaller than 1.0 for a nozzle with a taper edge. This explains well the instability of measurement concentration for Psi < 1.0, because of the multiple limiting trajectories.